During the reciprocal transmission of signals through transmitting channels and in particular through very long lines it is necessary, due to the attenuation which is produced on the transmitting channel or on the line, to insert amplifiers at regular intervals, with which amplifiers the respectively received signals are upgraded and amplified, so that they can then be further transmitted undistorted or with an amplified amplitude. As long as such amplifiers are only used for a transmitting device, their structure is relatively unproblematic. However, if such amplifiers are used in a bidirectional transmission, one must be sure that the signals which are emitted by one amplifier and which must be transmitted in a pregiven direction do not overdrive the input of a further amplifier, which is provided for the amplification of signals which arrive from this direction.
To avoid such damaging effects, filter arrangements are already known in communication systems in the form of the socalled switch circuits, with which it is possible to evaluate the transmitting direction of signals by their frequency and to assure in this manner that the outgoing signals do not reach the input of an amplifier which is provided for the arriving signals. However, such arrangements are useless, if for example digital signals are being transmitted, the frequency of which is the same for both transmitting directions. Furthermore the use of filter arrangements is associated always with an additional damping and signal distortion, so that the utilization of such arrangements in particular in the transmission of digital signals is not possible.
Therefore the purpose of the invention is to provide a circuit arrangement which operates without any filter of the common type and assures, independent from the frequency, the blocking of the input of a bidirectional amplifier against outgoing signals.
A circuit arrangement of the abovementioned type is constructed inventively to attain said purpose by the outgoing signals controlling a circuit for the continuous production of a reference voltage, which is fed to a comparator circuit which is arranged in front of the input for the purpose of a comparison with voltage which is there produced by the outgoing signals, and by the production of the reference voltage being controlled with respect to magnitude and phase corresponding with the voltage produced by the outgoing signals by means of a controlled condition, which is produced depending on a signal voltage, which is emitted, if necessary, by the comparator circuit.
The invention is based on the recognition that it is possible to compensate an amplified signal, which is emitted by a bidirectional amplifier, at the input of the bidirectional amplifier, which input is provided for the arriving signals, so that it cannot override this input, but that only arriving signals can be switched active at this input. This makes it possible to transmit signals on a transmitting channel simultaneously in two directions and to feed these through a bidirectional amplifier. Through the reference voltage, which is continuously produced in the inventive circuit arrangement from the amplified signals which occur at the output of a bidirectional amplifier and their adjustment to the voltage which is produced by these signals, it is possible to achieve a compensation effect for the outgoing amplified signals at the input of the bidirectional amplifier, because in the case of a perfect adjustment or "compensation" the output voltage of the comparator circuit assumes the value zero and therefore the input cannot be overdriven. A controlled condition can be derived from a voltage which is emitted, if necessary, by the comparator circuit and the reference voltage can be measured always exactly at any moment with respect to magnitude and phase so that it corresponds with the voltage which is produced by the outgoing signals and which receives certain distortions caused by the connected line or by its capacitive component and line attenuation. This permits an exact control to be possible independent from the length and the electric values of the connected line such that the difference between the outgoing signals and the comparison voltage receives always the value zero. This operation can take place automatically so that the effect is achieved that signals can be transmitted simultaneously in two directions, because only the outgoing signals are made ineffective at the input of the bidirectional amplifier. The invention utilizes thereby the fact that the outgoing signals practically never correspond with the arriving signals, so that the production of the reference voltage can be controlled by means of a controlled condition, which is not derived continuously, but at timed intervals, whereby these timed intervals can be controlled by the outgoing signals. This method makes it possible to cancel the influence of arriving signals on the mentioned comparison operation, so that the reference voltage compensates indeed only just the outgoing signals at the input of the bidirectional amplifier.
The circuit arrangement can advantageously be further developed such that the circuit for producing the reference voltage is a network which is connected to the line termination circuit for the line and which is connected to the bidirectional amplifier, which network contains variable resistors for reproducing the capacitive and the ohmic or resistive characteristics of the line. Through such a network, it is possible to reproduce the two line conditions which mainly distort the outgoing signals to a certain degree, so that the compensation voltage receives practically the same changes as the outgoing signals. This circuit principle can be accomplished very easily with a resistance network.
The comparator circuit may be a control transistor, which is arranged in front of the mentioned input, preferably a MOS field effect transistor, which is controlled at the gate electrode by the reference voltage and the operating current circuit which is connected to the potential existing on the connected line. Through this it is achieved that only one single control element is needed for the comparison, because if the reference voltage which is fed to the gate electrode corresponds with the potential which exists on the connected line and which is produced by the outgoing signals, the control transistor is blocked, so that at its output, as a compensation result, the voltage is emitted with the value zero, when the two supplied control magnitudes correspond exactly.
To transmit digital signals, the circuit arrangement according to the invention is preferably constructed so that for adjusting the variable resistors, two control loops, which each contain an integral control circuit, are provided and that the first control loop is switched active for a predetermined time prior to the rising flank of each one outgoing signal and for the same time after the rising flank of each one outgoing signal and the second control loop for the remaining time of this signal. This embodiment of the invention makes it possible in a particularly simple manner to carry out the above-described control operations in dependency of the outgoing signals in timely intervals. Due to the fact that the first control loop is switched active for a predetermined time prior to and for the same time after the rising flank of each one outgoing signal, there occurs a controlling of the distortions of the outgoing signals, which distortions are produced mainly by the capacitive and resistive characteristics of the connected line. Since the second control loop is switched active for the remaining time of the respective signal, it is possible to carry out during this time a control of the attenuation of the emitted signals, which attenuation is caused by the resistive characteristics of the connected line.
Of course it is also possible to process for example sinusoidal signals with the last-described embodiment of the invention, if the produced or amplified or also arriving signals are being transformed correspondingly for processing in a digitally operating circuit arrangement.
A particularly advantageous further development of the invention is characterized by the control transistor being associated with a circuit which is arranged in front of the mentioned input for the galvanic decoupling and serving as a modulation element for a high frequency oscillatory circuit which is provided within said circuit and which is part of a decoupling transmitter. Since between the amplifier stages or reception inputs of a bidirectional amplifier and the respectively connected line a potential-free coupling is needed, in this further development of the invention the control transistor which forms the comparator circuit is used simultaneously as an active element with which the galvanic separation between the line and the respective amplifier input is possible by modulation of the high-frequency oscillatory circuit. The control transistor which is used as the modulation element receives thereby an operating voltage so to speak from the voltage which is produced at the high frequency oscillatory circuit by feeding in a high-frequency oscillation, so that no separate operating voltage is needed.